nodes/attributes support in to_compatible_types

This extends XPath::Conversion.to_compatible_types so that it can also
take XML::Node and XML::Attribute objects as input.

lib/oga/xpath/conversion.rb

@@ -10,11 +10,11 @@ module Oga
       # @return [Array]
       #
       def self.to_compatible_types(left, right)
-        if left.is_a?(XML::NodeSet)
+        if left.is_a?(XML::NodeSet) or left.respond_to?(:text)
           left = to_string(left)
         end
 
-        if right.is_a?(XML::NodeSet)
+        if right.is_a?(XML::NodeSet) or right.respond_to?(:text)
           right = to_string(right)
         end
 

spec/oga/xpath/conversion_spec.rb

@@ -12,6 +12,26 @@ describe Oga::XPath::Conversion do
       right.should == 'bar'
     end
 
+    it 'returns two Strings when using two Nodes' do
+      n1 = Oga::XML::Text.new(:text => 'foo')
+      n2 = Oga::XML::Text.new(:text => 'bar')
+
+      left, right = described_class.to_compatible_types(n1, n2)
+
+      left.should  == 'foo'
+      right.should == 'bar'
+    end
+
+    it 'returns two Strings when using two Attributes' do
+      n1 = Oga::XML::Attribute.new(:value => 'foo')
+      n2 = Oga::XML::Attribute.new(:value => 'bar')
+
+      left, right = described_class.to_compatible_types(n1, n2)
+
+      left.should  == 'foo'
+      right.should == 'bar'
+    end
+
     it 'returns two Strings when using a NodeSet and Float' do
       set = node_set(Oga::XML::Text.new(:text => 'foo'))